The present invention relates to methods and apparatus for locating incipient faults in electric power distribution cables.
As discussed at length in U.S. Pat. No. 4,887,041 granted Dec. 12, 1989 to Matthew S. Mashikian, Robert B. Northrop, Rajeev Bansal and Chrysostomos L. Nikias and entitled "Method and Instrumentation for the Detection, Location and Characterization of Partial Discharges and Faults in Electric Power Cables", insulated underground power cables are extensively used to distribute electrical energy. Power cables which are used at elevated voltages, basically consist of a central conductor surrounded by a thin concentric layer of a semi-conducting material which is referred to as a conductor shield, a concentric layer of insulating material such as oil impregnated paper, polyethylene, or other polymer, and a second concentric semi-conducting layer referred to as an insulation shield, and a metal conductor applied in the form of a thin helically wound concentric cylindrical wires or flat strips which provide a ground or neutral. This assembly may be covered by a thin electrically insulating jacket. Power cables designed to operate below three thousand volts and communication cables may omit semi-conducting shields and neutrals.
As discussed in the aforementioned patent, such insulated cables are intended to operate safely and effectively over lifespans exceeding thirty years. However, because of manufacturing defects, installation errors, or aging under adverse conditions, cables may develop incipient faults which are evidenced by a "partial discharge" (PD) at the point of the incipient fault. Because of a defect developing within or adjacent to the insulation of a cable, intermittent arcing (partial discharge) occurs within the insulation. As used herein, the term "recipient fault" refers to a fault within the cable which will not cause immediate failure but which may lead to eventual failure, and the term "partial discharge" refers to the intermittent arcing which will occur at such a point particularly when a large excitation voltage is applied to the line. The energy involved in this partial discharge is extremely small, and, depending upon the type of insulating material, localized deterioration of the material occurs. Eventually, such incipient faults will produce a complete breakdown of the cable. Should this occur during a critical period, such as during a period of peak demand, there can be considerable customer inconvenience.
The location of faults in power lines has been the subject matter of prior patents and various technical papers. U.S. Pat. No. 2,628,267 measures the time of arrival of surge currents and reflections emanating from a ground along the cable. U.S. Pat. Nos. 3,244,975 and 3,255,406 disclose what is referred to as an impulse reflection method of testing cables where the time difference of detection of transmitted and reflected pulses are measured to determine fault location. U.S. Pat. No. 4,104,582 measures the time by clocking a counter. U.S. Pat. No. 4,491,782 discloses a technique of recording impulse reflection where impulse reflection signals are measured during normal operation and also under fault conditions, and it makes a comparison after fault occurs to determine the location of the fault in a cable.
In the aforementioned Mashikian et al U.S. Pat. No. 4,887,041, there is disclosed apparatus and methods for locating an incipient fault along a power transmission line using filters and amplification to modify the high frequency pulses on the line resulting from an excitation voltage and these modified pulses are converted into digital data interpreted by a computer to locate the point of the incipient fault along the length of the power line. Although this method and apparatus is effective, electrical noise in some installations may generate signals which are greater in amplitude than the signals from the fault and produce unreliable triggering of the digital storage oscilloscope used to record the data. In residential communities where underground electric power distribution cables are most widely used, electrical noise occurring within the frequency spectrum of the PD signals is predominantly from amplitude modulated (AM) radio broadcasts (550-1650 kHz). If the signal to noise ratio is not sufficiently high, the apparatus and method of the Mashikian et al Patent has not been uniformly reliable.
It is an object of the present invention to provide a novel method for detecting and approximating the position of an incipient fault in an electrical power transmission line wherein there may be substantial electrical noise.
It is also an object to provide such a method which will function effectively in environments which produce electrical noise signals of greater strength than the signals generated by the incipient fault.
Another object is to provide a method which is nondestructive, rapidly employed and reasonably accurate.
A further object is to provide novel apparatus for practicing the method which is relatively easily transported and operated.